SBInet -- also known as the "virtual fence" -- was to be the high-tech answer to stopping illegal immigration from Mexico, and in 2006 it was announced with great fanfare.

"The American people are rightfully insistent on the fact that we solve this 30-year-old problem," Homeland Security Secretary Michael Chertoff said at the time. "And this is about a solution which we believe is going to do the job."

After four years and $850 million dollars already spent, Secretary of Homeland Security Janet Napolitano suggests the virtual fence is turning out to be a high-tech lemon, and may be scrapped entirely.

"We know that we cannot continue to put out millions and millions of dollars of taxpayer's money if we're not confident that it's really not going to work," Napolitano said Friday.

If completed, the virtual fence will cost nearly a billion dollars.

The system is designed to use radar that detects illegal immigrants crossing the border, who would then be picked up by remote cameras and monitored by border patrol agents.

There have been a number of problems with the virtual fence from the beginning: the cameras often provided blurry images, the radar system performed poorly in bad weather, and it displayed false detections that were unable to distinguish between humans, cars and animals.

There were also cost overruns, while the primary contractor, Boeing, repeatedly missed deadlines, according to officials.

The system had so many technical problems that one congressional investigation concluded the system might be inoperable for two to seven weeks per year.

"Although some of the individual components perform acceptably, SBInet has failed to deliver on the promise of an integrated system that detects and simultaneously identifies intruders, and then quickly and accurately directs resources to intercept them," said T.J. Bonner, president of the National Border Patrol Council.

"This is due to unrealistic expectations about the capabilities of current technology, as well as a flawed underlying concept. In their quest to automate border security, the system's proponents appear to have lost sight of the fact that surveillance technology is incapable of apprehending anything; well-trained and highly-skilled law enforcement officers are necessary to accomplish that task."

But a source close to the SBInet program said the criticisms are dated and unfair, maintaining that SBInet is now a highly-effective tool in helping to secure the border.

Boeing declined to answer any questions on the record, but did provide this statement to ABC News:

"Boeing has worked closely with Customs and Border Protection to deliver capabilities that are in the hands of Border Patrol agents right now, providing them greater safety, situational awareness and resource effectiveness than ever before. Boeing stands behind its work on the SBI net system as a reliable, effective border security tool."

Homeland Security froze payments on the contract in March and has ordered a review of the project to determine whether the program should be completed or scrapped. The final report is expected to be released within two weeks.